A variety of devices are known for determining the orientation, or attitude, of an object. Such devices may include gyroscopes, motion reference units, horizon sensors, orbital geocompasses, star trackers, and the like. One such category of devices, referred to as sun sensors, are intended to allow the object's angle relative to the sun to be determined based on shadows cast onto a photosensor. For spacecraft such as satellites and space vehicles, which may have relatively unobscured views of the sun during certain periods of the day, sun sensors may provide a convenient and relatively simple way of determining the spacecraft's angle relative to the sun, based on which the attitude of the spacecraft may be controlled.
FIG. 1 schematically illustrates a perspective view of an exemplary previously known sun sensor 100. Sun sensor 100 includes photosensor 110, mechanical spacer 120, and aperture plate 130 having a round aperture 140 defined therein. Mechanical spacer 120 surrounds aperture plate 130 (only a portion of spacer 120 being illustrated), and supports and positions aperture plate 130 relative to photosensor 110. As the angle of sun sensor 100 varies relative to the sun, aperture plate 130 transmits sunlight 150 onto different regions of photosensor 110 (designated by the rounded, dashed line), and at the same time casts shadow 160 onto other regions of photosensor 110. Based on the relative proportion of sunlight 150 to shadow 160 on the different regions of photosensor 110, photosensor 110 generates one or more electrical signals that permit determination the angle of sun sensor 100 relative to the sun to be determined using known methods. Sun sensor 100 illustrated in FIG. 1 is a biaxial sun sensor because photosensor 110 includes four photodetectors (designated by the crossed, dotted lines) each respectively configured to receive an amount of sunlight 150 through aperture 140 and to generate a signal having a magnitude that is proportional to the amount of sunlight falling on the sensor. Based on the relative signals of the four photodetectors, the angle of sun sensor 100 relative to the sun may be determined in lateral dimensions x and y.
The sensitivity of sun sensor 100 to angle is a function of the relative dimensions of certain components of the sun sensor. Specifically, photosensor 110 may have a width 2L, where L is the width of each photodetector therein; mechanical spacer 120 may have a height h; and aperture 140 may have a diameter D. As the angle of sun sensor 100 changes relative to the sun, the rate at which the amount of sunlight 150 respectively falling on each of the photodetectors depends on the ratio between D and L. For example, the smaller the D/L ratio, the more quickly the amount of sunlight 150 changes as a function of angle on each of the photodetectors. The rate at which the amount of sunlight 150 respectively falling on each of the photodetectors also depends on the ratio between h and L. For example, the larger the D/L ratio, the more quickly the amount of sunlight 150 changes as a function of angle on each of the photodetectors. Those skilled in the art of sun sensors suitably may select appropriate absolute and relative values of D, h, and L for a given application.
In alternative previously known sun sensor 101 illustrated in FIG. 2, alternative aperture plate 131 includes a plurality of apertures 141 through which a plurality of sunlight regions 151 fall on respective regions of photosensor 111, with shadow 161 cast upon other regions of photosensor 111. Photosensor 111 may be a CMOS (complementary metal-oxide-semiconductor) imager that includes a relatively large number of photodetectors (pixels), e.g., greater than 100,000 or greater than 1 million photodetectors. An image of the sunlight and shadow cast onto photosensor 111 through apertures 141 is obtained, and the centroid of each illuminated region is calculated based on pixel coordinates. The centroids are digitally averaged together, applying geometric distortion factors determined by calibration, to generate two orthogonal solar incidence angles. Megabytes of storage memory and a 16-bit or larger microprocessor are typically required to process the image data. As for sun sensor 100, the sensitivity of alternative sun sensor 101 is a function of the relative dimensions of certain components of the sun sensor, such as the size of photosensor 111, the size of apertures 141, and the height h of the mechanical spacer disposed between aperture plate 131 and photosensor 111 (not illustrated in FIG. 2).
The various components of sun sensor 100 illustrated in FIG. 1 and alternative sun sensor 101 illustrated in FIG. 2 may be mechanically arranged relative to one another. For example, sun sensor 100 may be constructed by positioning and affixing mechanical spacer 120 around photosensor 110, and by positioning and affixing aperture plate 130 onto mechanical spacer 120 in such a manner that aperture 140 is positioned over photosensor 110. Such a mechanical coupling of components for each individual sun sensor may be time-consuming, and practical considerations may require that the components have a size sufficiently large to permit their mechanical handling. For example, sun sensors 100, 101 may have dimensions of 1 to 6 centimeters on a side. The amount of power required to operate the sun sensor, e.g., to apply a suitable bias to photosensor 110, may also scale with the physical size of the sun sensor. Accordingly, larger sun sensors may require larger power supplies, thus further increasing the mass of the sun sensor.
Furthermore, such mechanically coupled components may be susceptible to alignment errors. Although each individual sun sensor may be calibrated to reduce such errors, such individual calibration may be relatively time-consuming. Because of the comparative complexity of manufacturing, assembling, and calibrating such previously known sun sensors, the costs of such sun sensors may be on the order of thousands of dollars or more.
Thus, what is needed is a sun sensor with improved ease of manufacture, reduced cost of manufacture, reduced size, reduced power consumption, reduced susceptibility to alignment error, and reduced need for individual calibration.